1. Field of the Invention
The present invention relates in general to new modified porous polymeric materials, useful as membranes in ultrafiltration and microfiltration processes, having advantageous flow characteristics, and a process for preparing such materials.
2. Description of the Related Art
Porous polymeric materials are often used in the filtration industry for filtering gases and liquids. In many applications of filtration technology, it is desirable to utilize a membrane filter, which is mechanically strong, is thermally stable, is relatively inert chemically and is insoluble in most organic solvents. Often, it is desirable that the membrane have surface properties that are radically different from the properties inherent in the polymeric material. Desirable surface properties include wetability and sufficient flow properties.
Porous polymeric materials can be modified by coating with, or grafting to, another polymer which possesses more desirable properties that the first. These types of modifications are often referred to as xe2x80x9csurface modificationsxe2x80x9d or xe2x80x9csurface coatings,xe2x80x9d and are used to add properties to the bulk material that it does not otherwise possess.
Acrylate monomers have been widely used in polymerization reactions on the surface and in the matrices of porous polymeric xe2x80x9csubstratesxe2x80x9d to impart desirable qualities to otherwise less useful filter materials. One example of such a desirable quality is to treat a hydrophobic filter material to render it hydrophilic. Hydrophobic (water-fearing) filter materials will not wet with water, whereas hydrophilic (water-loving) filter materials will, and xe2x80x9cwetabilityxe2x80x9d is a desirable quality since a majority of filtration applications encounter aqueous based solutions.
One possible way to impart desirable filter characteristics (such as hydrophilicity) to an acrylate monomer is to cause the chains in the substance to cross-react. Acrylate monomers react with one another based on a xe2x80x9cfree radicalxe2x80x9d addition reaction mechanism and traditionally require a reaction initiator to start the chain growth process. These initiators are also referred to as free-radical initiators and are activated using some form of radiant energy.
U.S. Pat. No. 4,618,533 to Steuck discloses the use of acrylate monomers and xe2x80x9cthermalxe2x80x9d initiators to modify a polymeric membrane material. This patent requires the use of a free radical initiator and lists several compounds that may be employed as such, including persulfate, azo, and organic peroxy compounds. The initiator is a critical component because it forms free radicals when energy is added to the system in the form of heat, ultraviolet light, gamma or electron beam radiation and thus initiates the polymer chain growth process. Different initiators are specific for the type of energy sources used.
U.S. Pat. No. 4,886,836 to Gsell covers methods for xe2x80x9cactivationxe2x80x9d of membrane surfaces. Gsell suggests gamma radiation is preferred as an energy source for activation of a membrane surface, probably because of the high energy level of gamma radiation. Gamma radiation provides such high energy that free radical initiators are not required. In using gamma radiation, free radicals are created everywhere, even in the monomers present. The phenomenon, in which the monomers are attached only to other monomers, is referred to as xe2x80x9chomopolymerization.xe2x80x9d A gamma source, however, is very expensive and requires safety controls, and may not be economically feasible to many. Electron beams may also be used as a radiation source, but the equipment to generate such energy is also very expensive.
U.S. Pat. No. 5,468,390 to Crivello, Belfort, and Yamagishi covers treating polyarylsulfone with acrylates and UV light to reduce the protein binding characteristics of this polymer. xe2x80x9cSensitizers,xe2x80x9d free radical initiators, are not required because the polymer actually degrades to form free radicals and initiates the polymerization process. The mechanism of cleaving the polysulfone polymer chain is discussed in detail in this patent. The acrylic polymer is bound to the polysulfone membrane substrate by irradiating the sample for 3-5 minutes. Crivello et al. describe using a Southern New England Rayonette Irradiator equipped with sixteen low pressure mercury arc lamps with a broad emission at approximately 251 nm. In this example, the exposure time was relatively lengthy, at 5 minutes. Another experiment used a 450 Watt medium pressure Hanovia Inc mercury arc lamp for 3 minutes. Crivello et al. do not cover other polymer substrates; only polysulfone and polyethersulfone.
In view of the foregoing, one object of the invention is to provide a method for modifying properties of a polymeric material using monomers without the use of free-radical initiators.
A second object of the invention is to provide a method for modifying properties of a polymeric material without the use of expensive capital equipment.
A third object of the invention is to provide a method for modifying properties of a polymeric material where the exposure time to the ultraviolet light is less than one second.
A fourth object of the invention is to provide a method for modifying properties of a polymeric material where the intensity of ultraviolet light causes surface modification of the polymeric material.
A fifth object of the invention is to provide a method for modifying properties of a polymeric material where the modifying material is bound to the polymeric material.
A sixth object of the invention is to provide a method for modifying properties of a variety of polymeric materials.
A seventh object of the invention is to provide a modified polymeric material having hydrophilic properties and suitable for use as a membrane in a filtration process.
The above objectives are achieved by a process for modifying a porous polymeric material which includes placing the polymeric material in contact with a solution containing at least one type of acrylate monomer and exposing the polymeric material to ultraviolet light such that the intensity of ultraviolet light applied to the polymeric material causes surface modification by polymerization of the acrylate monomer without the use of a free radical initiator.
Using the process of the present invention, several advantages are realized. For example, no free radical initiator is used in the process of the invention. One resulting advantage is that the chemicals used in the invention are less expensive. It is apparent from the mechanism of free radical polymerization that free radicals do indeed form and are required for the polymerization to occur. The free radicals, however, are created from either the membrane and/or the monomer solution. In addition, conventional photo curing processes require photoinitiators, which are used with UV light. Initiators are often the most expensive part of the formulation, even though they are generally used in only small amounts. For example, Sartomer Company (Exton, Pa.) manufactures monomers and photoinitiators used for a variety of coating processes and applications. Sartomer manufactures and sells KIP100F, a photoinitiator compatible with UV light, at a cost of $17/lb, which is 8-9 times the cost of monomers. For example, hydroxypropyl acrylate (HPA), a monomer from Rohm America, costs approximately $2/lb when purchased in drum quantities.
Another advantage to eliminating initiators is that there is one less component in the system. This is a desirable aspect for toxicity considerations for the final product as well as for creating fewer variables in the manufacturing process. Also, once the initiator has been added to a monomer system in conventional methods, the solution has the potential of being less stable and may require special handling.
Another advantage of the present invention is that the process permits less expensive capital investment. The equipment used for the generation of ultraviolet (UV) light is relatively inexpensive, that is, on the order of tens of thousands of dollars as compared to the cost of the equipment needed to generate gamma or electron beam radiation. For example, one piece of equipment that can generate an electron beam was quoted at more than $800,000.
Moreover, the invention provides a faster reaction time. The total exposure time to UV light is approximately 0.5 seconds. By comparison, other UV treatments referenced herein require an exposure of 3-5 minutes.
The process of the invention also allows the modification and use of a variety of polymeric materials including polyvinylidenefluoride (PVDF), polyvinylidenedifluoride/polytetrafluoroethylene (PVDF/PTFE) fluorocopolymer, polytetrafluoroethylene (PTFE), polypropylene (PP), polyethylene (PE) and polyacrylonitrile (PAN) in the form of membranes, films or porous webs.
In addition, in the process of the invention, the acrylic polymer becomes bound to a membrane or other substrate. This is demonstrated by the fact that the hydrophilic treatment does not rinse out of the polymer even when washed with a variety of chemicals.
Furthermore, the resulting modified porous polymeric material exhibits hydrophilic properties rather than its original hydrophobic properties, except for PAN, which actually became more hydrophobic.
These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying Figures that illustrate, by way of example, the principles of this invention.